I personally always thought "third world" had more or less the same meaning as "third estate". That is, the "commoners"; or the ones left out of the global elit

I personally always thought "third world" had more or less the same meaning as "third estate". That is, the "commoners"; or the ones left out of the global elite; and I have yet to find anything that credibly proves that it doesn't come from there. On the upside; that meaning is the one usually associated with it, in my experience.